# Real-time monitoring via second-harmonic interferometry of a flow gas cell for laser wakefield acceleration

## TL;DR

This paper demonstrates real-time monitoring of gas density in a flow cell for laser wakefield acceleration using second-harmonic interferometry, enabling precise control for stable plasma interactions in LWFA applications.

## Contribution

It introduces a second-harmonic interferometry method for real-time density measurement in LWFA gas cells, improving control over plasma conditions.

## Key findings

- Real-time density monitoring is feasible with second-harmonic interferometry.
- Adjusting backing pressure and orifice size allows fine-tuning of gas density.
- Stable density control supports practical LWFA implementations.

## Abstract

The use of a gas cell as a target for laser weakfield acceleration (LWFA) offers the possibility to obtain stable and manageable laser-plasma interaction process, a mandatory condition for practical applications of this emerging technique, especially in multi-stage accelerators. In order to obtain full control of the gas particle number density in the interaction region, thus allowing for a long term stable and manageable LWFA, real-time monitoring is necessary. In fact, the ideal gas law cannot be used to estimate the particle density inside the flow cell based on the preset backing pressure and the room temperature because the gas flow depends on several factors like tubing, regulators and valves in the gas supply system, as well as vacuum chamber volume and vacuum pump speed/throughput. Here, second-harmonic interferometry is applied to measure the particle number density inside a flow gas cell designed for LWFA. The results demonstrate that real-time monitoring is achieved, and that using low backing pressure gas (< 1 bar) and different cell orifice diameters (< 2 mm) it is possible to finely tune the number density up to the range well suited for LWFA.